248 CHEMICAL ENGINEERING
conversion into hourly tonnage is consequently 0.033 or the tonnage per foot of width is 13.38 tons per hour. In round figures 13 tons.
Length of Screen.—Por screening capacity breadth is called for rather than length. Greater breadth than length means increased head room for uniformly distributing the material fed across the entry end of the screen and increase of the disadvantage under which flat screens labor, the difficulty of spreading the material fed at the entry point of the screen, A length greater than 5 ft. is unnecessary and unless the screen is heavily loaded 4 ft. is ample, these lengths give the capacity on rocks and ores per square foot as 3.25 and 2.60 tons per hour respectively for material whose maximum size is 1 in.
In arriving at the practical figures for capacity extremes have been assumed. For the ordinary shaking screen 300 strokes per minute is about the limit at which the screen frame will withstand the racking actions tending to destroy it. The assumption that the terminal grains have a range only between % in. and 1 in. is contrary to theory and practice. The average terminal grain would be very much smaller than this even with the most precise screening. Down to sizes of H-in. grain and smaller and in capacity problems trommels will be found better suited than flat screens. The field of the flat screen is in the finer sizes or where friability, as in grading salt, renders the revolving screen objectionable on the score of breakage.
It will now be seen that if other factors are equal the capacities of flat screens are proportional to the size of the material fed. If the capacity of inch rock and ore material on a %-in. screen is assumed to be 1 ton per hour per square foot of surface then its capacity on 6-mesh screen with 0.131-in. opening after passing through a 4-mesh opening with 0.185-in. aperture, all openings being assumed to be square, is about 0.17 tons per hour per square foot of screen. The meshes and sizes of apertures are the nearest equivalent to screen openings of 0.178 and 0.125 which are the exact equivalents of the screen ratio 1.414 beginning with 1 in. With the ratio 1.414 the reduction in aperture from size to size is by halving the area of the successively diminishing apertures. The tonnage figure is computed on the exact ratio, the inch aperture is very nearly six times the width of the theoretical aperture of 0.178 and the tonnage for this size consequently one-sixth that of the larger opening.
Objections to the Diameter Rule with Diminishing Size.—The principal objection to applying the diameter rule for capacity is that if the rate of progression of the material over the screen is the same for all the sizes of screen employed in a battery of screens, and as it usually is, the screen work becomes poorer as the size of the screen and material fed becomes smaller. If the strokes of the machine are diminished as the size fed diminishes the capacity falls off.
Vibrating Screens.—A number of new models of these devices have lately appeared on the market. The vibrating element is attached directly to the screen cloth and means are provided for changing the tautness of the cloth and regulating the number of vibrations per minute. In one type of these devices 2,000 to 3,000 vibrations are given by tappets actuated through gearing. In another kind a solenoid produces the desired vibrating effect and in a third an unbalanced electric motor is fastened to the screen at either end of its rotor shaft and gives a double rotary oscillation. The mechanism produces 3,600 oscillations per minute. The chief interest in these devices lies in the attempt to correct a short stroke by a large number of them thus tending to maintain capacity while at the same time maintaining good screening effect. They should be particularly valuable for fine screening dry loose materials. It would be